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Sandy intensifying, headed towards Jamaica

By: Dr. Jeff Masters, 2:49 PM GMT on October 23, 2012

Tropical Storm Sandy is on the move northwards towards Jamaica, and is slowly intensifying. The Hurricane Hunters are in the storm, and measured surface winds near 50 mph in the storm's northeast quadrant at 9 am EDT. The latest center fix, taken at 10:20 am EDT, found that Sandy's pressure had fallen 2 mb since early this morning, to 995 mb. Intermittent rain squalls from Sandy have been affecting Jamaica since Monday night, and Kingston, Jamaica has picked up 0.82" of rain from Sandy as of 9 am EDT. and Sandy is over very warm waters of 29.5°C, is in an unusually moist environment with relative humidities above 80%, and is experiencing light wind shear of 5 - 10 knots. These conditions are very favorable for intensification, and Sandy's heavy thunderstorms have increased in areal extent, intensity, and organization this morning, as seen on satellite loops.

Figure 1. Morning satellite image of Tropical Storm Sandy.

Sandy a potential threat to New England
Wind shear is forecast to be in the low to moderate range, 5 - 20 knots, through Thursday morning. This should allow continued development of Sandy, and rapid development is possible. The 8 am EDT SHIPS model forecast predicted a 62% chance that Sandy's winds would increase by 30 mph over a 24-hour period. However, the Hurricane Hunters found this morning that the center at 5,000 feet was displaced 7 miles from the surface center, and there was no indication that Sandy was building an eyewall, so I expect that the earliest Sandy could become a hurricane is Wednesday morning. The 5 am EDT NHC Wind Probability Forecast gave a 49% chance that Sandy would be a hurricane by 2 pm EDT Wednesday, when the center should be close to Jamaica. Wind shear will rise to a high 25 - 30 knots by Thursday and the storm will have to cross the rugged terrain of Cuba, which should weaken Sandy's winds by 20 - 30 mph. By Friday, Sandy should be over warm waters in the Central Bahamas, but only modest re-intensification by 10 - 20 mph should occur, due to high wind shear. At that point, a very complicated meteorological situation unfolds, as Sandy interacts with a trough of low pressure approaching the U.S. East Coast. The trough may steer Sandy northeastwards, out to sea, as predicted by the GFS, HWRF, and GFDL models. However, an alternative solution, shown by the ECMWF and NOGAPS models, is for the trough to inject a large amount of energy into Sandy, converting it to a powerful subtropical storm that hits New England early next week with a central pressure of 948 mb and winds near hurricane-force. The ECMWF and NOGAPS models are off by a full day in their predictions of when the storm would arrive, with the 06Z NOGAPS model predicting Sandy's heavy rains will arrive on North Carolina's Outer Banks on Sunday, then spread into the mid-Atlantic and New England on Sunday. The 00Z ECMWF model forecast predicts that Sandy will arrive in New England on Monday night. Stay tuned.

Figure 2. The Tuesday morning 06Z (2 am EDT) run of the GFS model was done 20 times at lower resolution with slightly varying initial conditions of temperature, pressure, and moisture to generate an ensemble of forecast tracks for Sandy (pink lines). These forecasts show substantial uncertainty in Sandy's path after Friday, with the majority of the forecasts taking Sandy to the northeast, out to sea, but a substantial number predicting a landfall in the Northeast or mid-Atlantic states of the U.S. The white line shows the official GFS forecast, run at higher resolution.

Figure 3. Morning satellite image of Tropical Depression Nineteen.

Tropical Depression Nineteen in the middle Atlantic
There has been little change to Tropical Depression Nineteen since it formed Monday in the middle Atlantic. TD 19 has a modest area of heavy thunderstorms, as seen on visible satellite images, and dry air from an upper-level low pressure system on its west side is slowing development. TD 19 is moving north-northeast at 15 mph, and has moved over waters that are 27°C, a full degree cooler what the storm had to work with on Monday. Wind shear is a moderate 10 - 20 knots, but is forecast to increase to the high range, 20 - 40 knots, on Wednesday. Ocean temperatures will drop below 26°C on Wednesday, so TD 19 has until Wednesday morning to get its act together and develop into Tropical Storm Tony. TD 19 will not threaten any land areas, and will likely be dead by Friday.

Video 1. Impressive weather video of the week: Baseball-sized hail splashing into a swimming pool during a hailstorm in South Africa on Saturday, October 20, 2012. The action gets interesting at about 1 minute into the video.

Tonight (October 23), PBS Frontline airs "Climate of Doubt", which examines the politics of climate change, and how a once generally accepted fact--that humans have contributed to, if not caused, global warming--has been dismissed by many politicians. It's on at 10 pm here in Michigan, but times may vary, depending upon the station.

Jeff Masters
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The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.